Thanks for all your lovely replies.
No it wasn't an easy thing to do, but I know they genuinely loved her & that they had only really given her up because they had been looking after their sons dog & didn't have the time she needed. I felt as sorry for them as I did for myself in a way...... They had always kept in touch to see how she was going so I guess I should have known this was a possibility.
We have already had an e-mail to report that she is doing well & they have been walking her & even found an off leash park nearby to take her to so that makes me feel better. They also had adopted a cat recently & so Holly has company now too as she loves cats. I really do think they are going to look after her or I would NEVER have agreed to give her back. We are definitely going to make sure to visit her too as often as we can.
My one ray of sunshine at the moment is that I have my 2 other babies to comfort me & my kids are pretty resilient as they have lost pets in the past. We had to put my Cairn terrier down a couple of years ago as he had started to lose his mind with old age & became agressive & we also lost our beloved Ragdoll cat last year to kidney disease, still it is harder to have to give one away.